Solar cells can be a viable energy source by utilizing their ability to convert sunlight to electrical energy. Silicon is a semiconductor material used in the manufacture of solar cells; however, a limitation of silicon use relates to the cost of purifying it to solar grade (SG).
Several techniques used to make silicon crystals for solar cells are known. Most of these techniques operate on the principle that while silicon is solidifying from a molten solution, undesirable impurities tend to remain in the molten solution. For example, the float zone technique can be used to make monocrystalline ingots, and uses a moving liquid zone in a solid material, moving impurities to edges of the material. In another example, the Czochralski technique can be used to make monocrystalline ingots, and uses a seed crystal that is slowly pulled out of a solution, allowing the formation of a monocrystalline column of silicon while leaving impurities in the solution. In yet another example, the Bridgeman process or heat exchanger techniques can be used to make multicrystalline and monocystalline ingots, and use a temperature gradient toward the central axis of the ingot in the direction of solidification to cause directional solidification. Such techniques utilize highly insulated ingot sides. Due to the lack of contact of the silicon with the crucible during the solidification, the dimensions of the ingot and the cost of the production compare to the Bridgman technique.
Various techniques utilizing temperature gradients for directional solidification of silicon can utilize a number of cooling or heating mechanisms. Such temperature control mechanisms can involve relatively expensive and difficult to maintain cooling or heating conduits within a directional solidification crucible.